1.Observations of sedimentary features, measurements of temperature and pH of waters, and sampling of siliceous sinter, thermal waters, gases, associated volcanic rocks, and microbes on and in siliceous deposits were done in thermal areas of Yellowstone National Park, Wyoming, USA.A degree of saturation with respect to amorphous silica increases through evaporation along the drainage channel, and controls the rate of silica deposition. Aluminum concentrations in siliceous deposits shows positive correlation with temperatures of thermal waters.2.Various bacterial assemblages were found in siliceous deposits of different temperature conditions from Cistern Spring, Norris Geyser Basin. Also, it is recognized that the rate of silica deposition correlates with the amount of thermophillic bacteria. This concides with the result obtained from a thermal pool at a geothermal power plant, suggesting impoartanc of a study of the mechanism of bacterial control on silica deposition.3.During the investigation at Yellowstone it is shown that the different thermophillic bacteria dominate in different temperature conditions. Then the fabric and structure of siliceous deposits in geologic past can tell the depositional environments. Ikiryu sinter, 0.5 Ma old deposit in Kuju volcanic area, was investigated as a case study. In addition, siliceous deposits formed in pipes at geothermal power plants were precisely observed.4.Inactive sinters at Beowawe Geyser area and Steamboat Springs were studied and samples were collected. Chemical analysis and study of structures for these samples are in progress..